Fabrication of single-crystal Fe-Mn-Al-Cu alloys by cyclic heat treatments

In general, single crystal shape memory alloys exhibit superior shape memory effect and superelasticity when compared to their polycrystalline counterparts. However, it is challenging and costly to prepare a single crystal in traditional ways, such as the Czochralski method and directional solidification technique. Recently, cyclic heat treatment (CHT) has emerged as a facile and cost-effective method to fabricate single crystal by inducing abnormal grain growth. In the present work, we aimed to fabricate Fe-Mn-Al-Cu single crystals utilizing the CHT technique. Single crystal Fe-Mn-Al-Cu shape memory alloys were successfully prepared, and the effect of CHT parameters on abnormal grain growth was investigated. It was revealed that increasing the cooling rate resulted in a non-monotonic response in the maximum grain size, which initially increased and was followed by a subsequent decrease. This behavior was attributed to the subgrain refinement, increase in subgrain misorientation angles, and the narrowing of the low density zones (LDZs) of subgrains. Decreasing the lower temperature of CHT caused similar trends in maximum grain size, due to the increase in subgrain misorientation angle and the expansion of the LDZs. Extending the holding time at the lower temperature of CHT had minimal impact on grain size. Increasing the heating rate of CHT resulted in a reduction in the maximum grain size because of the narrowing of the LDZs. Moreover, the addition of 2 wt% Cu led to a reduction of the maximum grain size, attributing to the decreased solvus temperature of the γ phase as a result of Cu addition. Consequently, the γ phase and subgrains were refined, and the LDZs were narrowed.